Thanks for the link, Matt.

I did not provide all info in my previous post, for simplicity.
In fact, my friend has been using his PC for months before he realized his archive directory was gone. Most of the recovery I succeeded in doing is actually from a backup disk, which gets rarely used, and which had been synched with his PC so that even that lost the Archive directory. So, it is not true that he had no backup. He actually had an incomplete backup strategy.

In any case, he is positive neither he nor his wife deleted the 500GB "Archive" directory. Such a big directory, moreover, would not fit in his Trashcan, so there would be way too many confirmations he would have to mistakenly OK, I think.
And, time to delete such a dir would also be quite alarming for him; he is actually experienced enough and computer savvy enough not to do anything so naive. He is an architect and works on his PC all day long, being usually very careful with backups. For his job stuff, of course. Not careful enough for his personal stuff.

I still can't understand what happened in the first place, anyway.